When energy consumption reaches peaks, especially in extreme cold, power shortage looms and grid operators face a dilemma: either imposing rolling blackouts on certain customers, which is tantamount to rationing and hence a last-resort measure, or buying the missing energy at exorbitant prices on the spot market. The reason for this problem is that bulk energy storage techniques are outdated and entail too many losses. The stakes are high, since the energy efficiency ideal will remain out of reach unless surplus energy storage is optimized. Here is an overview of the methods used and the breakthroughs to come [1].

Background

In most developed countries, average electricity supply exceeds average demand. This statement is only true in theory. In energy generation, totals and averages are irrelevant since electricity, like any sort of energy, is volatile; it is a good whose storage is imperfect. In other words, one cannot store electricity just like one stores corn.

The intermittent nature of wind and solar power is another essential part of the problem. Wind blows more after dark; however electricity needs are higher during daytime. Similarly, sun does not shine constantly. Smart energy storage enables to capture today’s excess energy to make it available tomorrow, it avoids wasting and consequently cuts costs; it gets us closer to the energy efficiency ideal.

Technical hurdles

Why not simply use giant batteries? It is unfortunately impossible, for batteries have a very short charge/discharge cycle and cannot deliver grid-scale performance. Bulk energy storage schemes are both simple and clever: unlike batteries, they don’t actually store energy but use off-peak excess power to generate energy available in the future.

PSH is quite simple as it merely takes advantage of local topography, using two elements: gravity, induced by a difference in altitude, and water. Excess energy produced during off-peak periods is used to pump water from the lower to the higher reservoir; at peak times, water flows back down and serves to generate power. Nowadays, this conventional system largely prevails over others; however its topographical constraints make its expansion difficult, since very few natural sites fulfil the necessary conditions.

Offshore PSH

The Danish architectural firm Gottlieb Paludan and the Technical University of Denmark launched an ambitious project called “Green Power Island”. The project involves building artificial islands combining wind and solar power and including a deep central reservoir and a wind farm nearby. Just like in old-fashioned PSH, off-peak energy – which happens to be generated when the wind blows the hardest – is used to pump water from the reservoir up to the sea, whereas at peak times, sea water is allowed to flow down the reservoir to drive turbines which generate power that is instantly available. Offshore PSH is based on the same principle as conventional PSH, minus the topographical constraints on the land.

Compressed-air energy storage (CAES)

This is the second most used technique, though far behind PSH (today there are only two plants of this kind, one in Huntorf, Germany, and another in the south of the US, in Alabama). Excess energy is used to compress air trapped in large containers and later released to drive turbines. The system’s main flaw is that air heats up when compressed, which causes energy losses, and cools down when released, which makes it necessary to heat it up again. With only a 42%-efficiency in Huntorf, CAES as developed today is not an optimal solution yet.

Isentropic, a company based in Cambridge (UK), designed a system using argon gas to convey heat between two tanks filled with gravel. Surplus energy drives a heat pump which pressurizes and heats up the argon, creating a temperature differential between the two tanks, with one at 500°C and the other at -160°C. At peak times, the heat pump works the other way, expanding and cooling down argon to generate power. Isentropic says its system is 72 to 80% efficient.

The Californian company BrightSource (Oakland) develops a technique called SolarPLUS which consists in storing heat in molten salts. Solar energy is used to boil water, which in turn drives steam turbines, whereas a heat exchanger transfers part of the heat to molten salts. BrightSource can deliver energy after dark, which makes it more flexible than conventional solar plants.

A promising market requiring significant regulatory arrangements

According to the Pike Research Institute, a market-research firm, 93 billion € could be invested in bulk energy storage by 2021. Most investments will go towards new forms of CAES. Governments in California, Germany and the UK, for instance, have created legal incentives. However, nothing has been done yet at the EU level.

Just as any technical evolution, bulk energy storage will inevitably entail profound legal changes. The problem is that the law discriminates between energy producers and grid operators (take the French example: EDF on the one hand and RTE on the other hand). Energy storage schemes can be used by both, which creates pricing and billing issues. Finally, it remains to be seen whether the costs related to bulk energy storage facilities will be passed on to users.